1. Field of the Invention
The present invention concerns a method to determine a background phase in phase values of a phase image data set and a device to determine the background phase.
2. Description of the Prior Art
The phase information of the magnetic resonance signal that describes the rotation of the local magnetization vector in the plane orthogonal to the B0 field direction can be used in many regards in magnetic resonance tomography (MRT). For example, the phase information contained in the MR signal can be used to separate adipose and aqueous tissue, for flow measurement, in susceptibility-weighted MRT and to determine temperature. In a method known as thermotherapy the temperature in tumor cells is specifically increased in order to kill the tumor cells or to make these more sensitive to accompanying therapy measures. For example, the tissue heating can hereby ensue via focused ultrasound or with the aid of lasers. A temperature monitoring of the heated tissue is required in order to not destroy the healthy tissue via the increased temperature. In addition to invasive temperature measurement with temperature probes placed in the heated tissue, some MR parameters (such as the chemical shift, the T1 relaxation time or the diffusion constant) can be used for non-invasive temperature measurement.
In temperature monitoring based on the temperature dependency of the chemical shift, the resonance frequency altered by the temperature change is detected as an additional phase shift of the complex MR signal in a voxel (image point). In temperature imaging based on the chemical shift only temperature changes can be shown, for example by taking the difference of two phase image data sets that were acquired at different temperatures. The phase image data set acquired at a known initial temperature hereby serves as a reference data set from which the subsequent phase image data sets are subtracted. These methods operating with reference data sets have the disadvantage that movements of the examination subject during the acquisition of the two data sets or other external interferences lead to phase changes that are incorrectly interpreted as temperature changes. Furthermore, the B0 field constancy over time and a current drift in the shim coils play a role since these also lead to phase changes in the detected signal that can likewise be incorrectly identified as temperature changes. In addition to these methods with reference image data sets there are techniques known as reference-less methods in which a temperature is concluded from only the measured phase values. These methods have the disadvantage that information about how the MR system-dependent background phase changes spatially over the image must be present. The phase position in a pixel is affected not only by the frequency of the magnetization in this pixel but also by system components, for example the RF receiver or the demodulator.
U.S. Pat. No. 7,505,805 describes a method to determine temperature with phase images using the reference-less approach.
In Magn Reson Med 2004 June; 51(6), P. 1223-31, Rieke et al, have described a method in which the phase information from an outer edge of an ROI (Region of Interest) is used in order to interpolate the background phase values in the heated regions. A polynomial of lower order was hereby used. However, there is no physical or mathematical basis for the use of such polynomials. The precision of the procedure is sufficient for small regions and for situations in which the background phase—i.e. the system-induced phase information—changes only slowly across the image. However, in regions in which the susceptibility changes a fast spatial change of the background phase occurs. The determination of the polynomial is likewise computationally expensive and can lead to problems in the calculation.
In Medical Imaging, IEEE Transaction on Volume 26, 6th Edition, June 2007, Pages 813-821, Rieke et al. have described the use of specific MR pulse sequences with which an independent imaging of fat and water is possible, wherein the fat signal is not temperature-dependent and therefore can be used to extract the background phase. However, a special anatomy in which sufficient fat is present is necessary for this. Moreover, the spectral line of the adipose signal is subject to changes from one location to another, which leads to different phase values over the different resonance frequencies, whereby problems occur in the determination of the background phase with the aid of the adipose signal portions.